Currently, proposals are made for perpendicular magnetic recording as an alternative to horizontal magnetic recording. In horizontal magnetic recording, recording dots have a magnetization direction along the recording plane in the magnetic recording layer. Perpendicular magnetic recording has an advantage over the horizontal magnetic recording in that recording density can be increased easily. In perpendicular magnetic recording, recording dots have a magnetization direction in the thickness direction in the magnetic recording layer. A magnetic disk apparatus suitable for perpendicular magnetic recording is described in e.g. Japanese Laid-open Patent Publication No. 2003-157507, in which the magnetic disk apparatus includes a bit patterned medium as a recording medium. In the bit patterned medium, magnetic regions each representing a recording dot are spaced from each other equidistantly.
Bit patterned media have a data recording area which is a non-magnetic area scattered with magnetic regions, and a servo-pattern area which is used for disk access control such as magnetic head positioning control and clock signal generation. The servo-pattern area is formed with a large number of belt-like magnetic regions extending substantially radially of the magnetic disk. In the data recording area each magnetic region is given a magnetization direction as a representation of a datum to be recorded whereas in the servo-pattern area all of the magnetic regions are given the same magnetization direction in a formatting procedure which is performed, generally, during the manufacturing process.
However, the magnetic disk apparatus equipped with the above-described conventional bit patterned medium has a problem in regards to disk access control, i.e., a problem that magnetic regions having a large area are susceptible to magnetization reversal caused by external disturbances.
As illustrated in FIG. 6A, a magnetic region 100 is composed of polycrystalline crystal grains and includes a plurality of magnetic domains 110-130, each of which is bordered by a crystal grain boundary and functions as a unit for generation of magnetization directions P1-P3. The magnetic domains 110-130 included in the magnetic region 100 have a strong magnetic exchange coupling force, and therefore the magnetic region 100 is magnetized in one direction.
FIG. 6B depicts a magnetic region 100′ which is larger than the region illustrated in FIG. 6A and therefore includes a larger number of magnetized magnetic domains 110′-150′. Take, for example, the middle magnetic domain 130′. This domain is influenced by a magnetic field MF which is generated by the sandwiching magnetic domains 110′, 120′, 140′ and 150′. The greater is the area of the magnetic region 100′, the greater is the influence from the magnetic field MF from both sides, leading to generation of a large demagnetizing field DF in each of the magnetic domains 110′-150′. This seems to suggest that the magnetic region 100′ has a smaller coercive force as its area becomes larger, and therefore the magnetization directions P1-P5 in a large magnetic region 100′ can be reversed easily by external magnetic disturbances.
This affects the disk access control servo-pattern area which contains a large number of magnetic regions which have a larger area than magnetic regions in the data recording area. When the disk is new, the magnetic regions have a perfectly uniform magnetization direction, but the magnetization direction is likely to be reversed by external disturbances and other forces in the magnetic regions. Once the reversing of magnetization direction occurs in the servo-pattern area, it becomes no longer possible to make correct magnetic recognition of the magnetic regions in the servo-pattern area, leading to troubles in magnetic head positioning control and clock signal generation, and to inability to perform the disk access control properly.